Field of the Invention
The present invention relates to a sheet detection device, a sheet conveyance device, and an image forming apparatus capable of detecting a position of a conveyed sheet.
Description of the Related Art
Generally, to match the timing for conveying a sheet to an image transfer position and the timing for conveying an image formed at an image forming portion to the image transfer position, an image forming apparatus includes in a sheet conveyance path a sheet detection device capable of detecting the position of the front end of a sheet. This sheet detection device can also be used to determine the arrival timing of a sheet or the length of a sheet, or detect a jam (paper jam).
Further, in the image forming apparatus, the recording position of an image relative to the position of a sheet is an important factor for image quality. Japanese Patent Application Laid-Open No. 9-183539 discusses an image forming apparatus in which a skew correction unit is provided to improve the accuracy of the recording position of an image. In this image forming apparatus, a skew correction unit using shutter members is disposed in a pair of conveyance rollers immediately before an image transfer portion. In or downstream of the skew correction unit, a sheet detection device for detecting a sheet is disposed.
Conventionally, as a pair of conveyance rollers immediately before an image transfer portion, there is a pair of conveyance rollers set to be able to convey a sheet without being influenced by the conveying speed of another pair of conveyance rollers disposed upstream of this pair of conveyance rollers. A sheet detection device is disposed in a nip portion of the pair of conveyance rollers or downstream of the pair of conveyance rollers, thereby, in synchronized timing with an image formed by an image forming portion, adjusting the speed of a sheet or outputting a signal for the image forming portion to start forming an image. As described above, the skew of a sheet is corrected by shutter members, conveyance is controlled based on detection by a sheet detection device, and the timing of image formation is controlled, whereby it is possible to achieve an image forming apparatus in which the accuracy of the recording position is high.
In recent years, an image forming apparatus has been demanded to further increase the number of sheets in image formation per unit time. As an example thereof, there is a method for reducing the distance between the rear end of a preceding sheet and the front end of a following sheet (hereinbelow referred to as a “sheet-to-sheet distance”). With the reduction of the sheet-to-sheet distance, a sheet detection device needs to increase its responsiveness to detect a short sheet-to-sheet distance.
In the apparatus discussed in Japanese Patent Application Laid-Open No. 9-183539, however, each shutter member in a registration shutter mechanism is set to rotate by a certain angle in a counterclockwise direction about a roller shaft from when the shutter member stands by for a sheet to when the sheet passes through the shutter member. After the rear end of a preceding sheet passes through the shutter member and until the shutter member returns to the standby state, a return time corresponding to the certain angle is required. For the length of this return time, it is not possible to cause a following sheet to enter the registration shutter. Thus, according to this configuration, the return time of the shutter member hinders the reduction of the sheet-to-sheet distance.